MDI is a proprietary image format used by Microsoft Office Document Imaging.
It is almost identical to TIFF, but has a different header, and may use one of several undocumented compression schemes.
Files begin with bytes
45 50 2a 00.
The compression schemes are reportedly defined as follows:
|34718||Binary Level Codec (BLC)|
|34719||Progressive Transform Codec (PTC)|
Other sources give the following, but 34719 and 34720 may have been accidentally swapped.